1. Field of Invention
The present invention relates to a manufacturing method capable of preventing the corrosion of a metal-oxide-semiconductor (MOS). More particularly, the present invention relates to a manufacturing method that uses a protective oxide layer capable of preventing corrosion during subsequent cleaning operations.
2. Description of Related Art
A MOS transistor is an important semiconductor device in an integrated circuit. The MOS transistor acts like a switch that can be switched ON or OFF through its voltage. Since the MOS transistor is a major component in the operation of the VLSI circuit, any deviation in the components' voltage sensitivity can lead to serious control problems.
FIGS. 1A through 1C are schematic, cross-sectional views showing the progression of manufacturing steps according to a conventional method of producing a MOS device.
First, as shown in FIG. 1A, a gate oxide layer 12 is formed over a silicon substrate 10. Then, a polysilicon layer 13 is deposited over the gate oxide layer 12. Next, a silicide layer 14 is formed over the polysilicon layer 13. Thereafter, a top cap layer 15 is deposited in sequence over the silicide layer 14.
Next, as shown in FIGS. 1B and 2, photolithographic and etching operations are conducted to pattern a gate structure 18. The gate structure 18 is composed of a gate oxide layer 12, polysilicon layer 13, silicide layer 14 and top cap layer 15.
Next, as shown in FIGS. 1C and 2, subsequently, using the top cap layer 15 as a mask, a first ion implantation is carried out, implanting ions into the substrate 10 to form lightly doped source/drain regions 16.
Thereafter, as shown in FIGS. 1D and 2, spacers 17 are formed on the sidewalls of the gate structure 18. Finally, using the gate structure 18 and the spacers 17 as masks, a second ion implantation is carried out, again implanting ions into the substrate to form heavily doped source/drain regions 11.
In general, the silicide layer 14 is formed using tungsten silicide. However, the tungsten silicide layer has some disadvantages as follows:
(1) Threshold voltage is affected: In a dual gate CMOS transistor, the P-type ions in the P-type gate and the N-type ions in the N-type gate can cross-diffuse into each other through the tungsten silicide layer in the thermal process. This cross-diffusion of different type ions causes a serious shift in the threshold voltage.
(2) Resistance is affected: The tungsten silicide has a higher sheet resistance which induces a serious delay in the response time of the MOS and reduces the operating speed of the integrated circuit.
For at least the reasons described above, titanium silicide is used to replace the tungsten silicide and to improve the yield of the MOS. However, a thin layer of polymer is formed on the surface of the substrate 10, after the ion implantation step is performed. The thin polymer layer reduces the electric sensitivity between the gate, drain region, and source region. Therefore, it is necessary to remove this thin polymer layer by using strong acidic etchant, such as hydrogen fluoride solution. Unfortunately, corrosion occurs on the silicide layer 14 during the removal process, as seen in FIG. 3 which seriously reduces the resistance of the gate.